In a wire electric discharge machine, a stretched wire, which is laid in a tensioned state, between a wire feed roller and a wire collection roller is travel.
During wire traveling, the tension of the wire is higher and the amount of stretching thereof is larger near the wire collection roller than near the wire feed roller due to the friction force of a guide roller placed between the wire feed roller and the wire collection roller. Therefore, the circumferential speed of the wire collection roller is generally higher than that of the wire feed roller.
Conventionally, when an acceleration/deceleration process is performed according to rotational speed instructions for the rollers during wire traveling, the acceleration/deceleration process is performed by independent control systems having different control performances and thus a difference in the speeds of the rollers during the acceleration/deceleration process differs from that immediately before the acceleration/deceleration process is performed.
If the circumferential speed of the wire collection roller becomes higher than that of the wire feed roller and a difference in the circumferential speeds increases to exceed a certain value, the wire is broken. Contrary thereto, if the circumferential speed of the wire feed roller becomes higher than that of the wire collection roller, the wire sags.
Accordingly, in a wire traveling device of a conventional wire electric discharge machine, when automatic wire connection is performed using a fine wire at a high speed, wire breaking or wire sagging frequently occurs during wire acceleration/deceleration processes. Therefore, at the time of automatic connection of a fine wire, the wire feed speed needs to be lower.
Patent Literature 1 discloses a technique of detecting rotational speeds of respective rollers and keeping the tension of wires constant.